Trystan Casey
The human body is a complex system that relies on electrical impulses to regulate movement. These impulses are generated by the nervous system and transmitted through muscle fibres, resulting in muscle contractions that enable movement. Understanding the electrical properties of these impulses is crucial for developing rehabilitative technologies such as Functional Electrical Stimulation (FES), which aids individuals with spinal cord injuries or strokes in regaining motor function. To measure and analyse these electrical impulses, techniques such as electromyography (EMG) and microelectrodes are employed. By recording EMG activity from able-bodied individuals, researchers can establish patterns of electrical stimulation that mimic muscle movements. This knowledge is essential for developing treatments and interventions that target specific muscles or muscle groups to restore movement in those with injuries or disabilities affecting their motor functions.
| Characteristics | Values |
|---|---|
| Method | Functional electrical stimulation (FES) |
| Purpose | To restore motor function in paralyzed individuals following spinal cord injury or stroke |
| Mechanism | Stimulating combinations of muscles with a specific temporal pattern to elicit useful motor responses |
| Measurement | Electromyographic (EMG) activity recorded from able-bodied subjects as a template for electrical stimulation |
| Transfer Function | Converts EMG signals into patterns of electrical stimulation by varying pulse frequency and amplitude |
| Evaluation | Recreate muscle active states associated with unloaded free movements of the distal segment of a body part, e.g., thumb |
| Results | Production of behavioral outputs (torque or displacement profiles) that closely match desired behaviors |
| Impulse Response | Inject a PRBS (pseudo-random binary signal) sequence, record the response, and correlate PRBS with system response |
| Frequency Response | Measure and calculate the frequency response, then determine the transfer function of the system |
| Action Potential | Depolarization of cell membrane, causing rapid entry of positively charged ions and a change in electrical charge |
| Resting Potential | Dependent on the concentration gradient of K+ ion across the cell membrane |
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What You'll Learn
Measuring muscle activity with electrical stimulation
The nervous system of animals and the control of muscle movement are governed by electrical interactions. Electrical stimulation can be used to mimic muscle activity and restore some degree of motor function in individuals who have sustained a spinal cord injury or stroke. This technology is known as functional electrical stimulation (FES).
FES devices stimulate muscle contraction through artificial stimulation by taking advantage of the retained electrical excitability of the motor axons that innervate most paretic muscles. This residual function allows for the induction of muscle contraction through artificial stimulation. By stimulating combinations of muscles with a specific temporal pattern, useful motor responses can be elicited.
To identify the spatio-temporal patterns of muscle stimulation needed to elicit complex upper limb movements, electromyographic (EMG) activity recorded from able-bodied subjects can be used as a template for electrical stimulation. EMG measures muscle response or electrical activity in response to a nerve’s stimulation of the muscle. During the test, small needles or electrodes are inserted through the skin into the muscle to pick up electrical activity, which is then displayed on an oscilloscope. The action potential (size and shape of the wave) that this creates on the oscilloscope provides information about the ability of the muscle to respond when the nerves are stimulated.
A generalized transfer function can be used to convert the recorded EMG signals into an appropriate pattern of electrical stimulation. This function maps EMG activity into a stimulation pattern that modulates muscle output by varying both the pulse frequency and the pulse amplitude. The stimulation patterns produced by this transfer function mimic the active state measured by EMG by reproducing complex patterns of joint torque and joint displacement.
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The role of electricity in the functioning of plants and bones
While electricity often evokes images of man-made technology, it is also integral to many life processes. The nervous system of animals and the control of muscle movement, for example, are governed by electrical interactions. Even plants rely on electrical forces for some of their functions.
Plants, for instance, use electricity for photosynthesis and respiration, increasing metabolism and facilitating growth and development. High voltages can ionize negative oxygen ions, playing a role in air disinfection and purification. Electricity can also activate antioxidant defence systems and change the synthesis of metabolites in plants. However, the impact of electricity on plant development and the accumulation of metabolites is not yet fully understood.
In the human body, electricity plays a role in the functioning of bones. Bones have piezoelectric properties, meaning they exhibit electric polarization in response to mechanical stress, and an applied electric field causes strain. The piezoelectric properties of bone are of interest due to their hypothesized role in bone remodelling. The magnitude of the piezoelectric sensitivity coefficients of bone depends on frequency, the direction of load, and relative humidity.
Additionally, bones respond to electrical stimulation. Functional electrical stimulation (FES) is a rehabilitation technology that can restore motor function in individuals with spinal cord injuries or strokes. FES takes advantage of the electrical excitability of motor axons that innervate paretic muscles, allowing for the induction of muscle contraction through artificial stimulation. By stimulating different muscles with specific temporal patterns, useful motor responses can be elicited.
Furthermore, bone health is influenced by a combination of genetic and environmental factors. Diet and physical activity are critical to bone health, and mechanical loading is essential for maintaining normal bone mass and architecture. Bones require adequate amounts of loading and weight-bearing to remain strong, following the principle of "use it or lose it."
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The electrical system of the heart
The heart is a pump made of muscle tissue. Like all muscles, the heart requires a source of energy and oxygen to function. The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart.
An electrical stimulus is generated by the sinus node (also called the sinoatrial node or SA node). This is a small mass of specialized tissue located in the right upper chamber (atria) of the heart. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The atria are then activated, and the electrical stimulus travels down through the conduction pathways, causing the heart's ventricles to contract and pump out blood.
The electrical impulse travels from the sinus node to the atrioventricular node (also called the AV node). There, impulses are slowed down for a very short period, allowing the atria to contract a fraction of a second before the ventricles so that blood empties into the ventricles before they contract. After passing through the AV node, the electrical current continues down the conduction pathway, through a pathway called the bundle of His, and into the ventricles. The bundle of His divides into right and left pathways (bundle branches) to stimulate the right and left ventricles.
The ability of cardiac muscle cells to drive the electrical impulse that triggers contraction depends on the resting potential, which is primarily dependent on the concentration gradient of the K+ ion across the cell membrane. To initiate contraction, a complex mechanism of ion channels in the cell membrane opens momentarily, allowing a rapid entry of Na+ into the cell. This causes the cell to become electrically positive, while the outside becomes negative. This process is called depolarization and results in the flow or movement of certain ions, especially sodium. The depolarization process also allows calcium to enter the cell, where it is responsible for the binding of actin and myosin, resulting in contraction.
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Functional electrical stimulation (FES)
FES devices can be current or voltage-regulated. Current-regulated FES systems deliver the same charge to the tissue, regardless of skin/tissue resistance, and hence do not require frequent adjustments of the stimulation intensity. Voltage-regulated devices, on the other hand, may require more frequent adjustments as the charge they deliver changes with skin/tissue resistance. Biphasic, charged-balanced pulses are used to improve safety and minimize adverse effects. Pulse duration, pulse amplitude, and pulse frequency are key parameters regulated by FES devices.
FES equipment comes in various forms, depending on the treatment location and desired outcome. A typical setup includes a small electrical box (neuromuscular electrical stimulator unit), wires that carry the electrical impulse, and electrodes that attach to the targeted muscles or nerves. Electrodes can be placed on the surface of the skin, under the skin, or fully embedded into the muscle. FES can be used to generate muscle contractions in paralyzed limbs, producing functions such as grasping, walking, and standing.
FES has been studied for its potential in treating multiple sclerosis (MS) symptoms, such as foot drop, where it can stimulate the nerve that normally lifts the front of the foot during walking. It has also been investigated for its role in improving swallowing, hand and arm function, and breathing problems in pulmonary disease patients. FES is generally well-tolerated, with the electric shock sensation causing discomfort but not pain.
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Measuring the electrical event of a single cell
The electrical event of a single cell can be measured to gain insight into complex physiological states and to aid in the understanding of certain disease processes. This is especially useful in cancer research and treatment, as well as in the development of new ideas for disease treatments.
There are several methods for measuring the electrical event of a single cell. One classical technique is the patch-clamp method, which was introduced in 1976 by Neher and Sakmann. This technique uses a glass micropipette as a probe to suck a cell membrane into the pipette, forming a high electrical resistance or giga-seal. The ion current that flows through the pipette, which contains an electrode, is then measured through an amplifier. The patch-clamp technique can be used to study and provide valuable information on the electrical properties of biological cells, including the analysis of ionic currents in the cell membrane under controlled conditions. It is a time-consuming process that requires skilled operators, but it offers high sensitivity and low-noise measurement of currents passing through ion channels.
Another method is the use of electro-biosensors, which measure the impedance of a cell grown on a surface embedded with an electrode. The impedance is influenced by factors such as the number, size, and shape of cells on the electrode surface, as well as the distance between the cells and the surface. This technique is useful for predicting the efficacy of treatments and detecting biased compounds and cellular toxicity in real-time.
Single-cell electrical manipulation is a safe technique that can be used to separate individual cells for specific analysis and to isolate and characterize rare cells. It enables the detection of various cellular components and the exploration of cell proliferation and cell cycles.
Additionally, functional electrical stimulation (FES) is a rehabilitation technology that can restore motor function in individuals with spinal cord injuries or strokes. FES takes advantage of the electrical excitability of motor axons and uses electrical stimulation to induce muscle contraction and useful motor responses. This technique helps identify the patterns of muscle stimulation needed to elicit complex movements.
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Frequently asked questions
An electrical impulse is a very short, strong burst of electrical energy. In the context of movement, electrical impulses are involved in the nervous system of animals and the control of muscle movement.
One way to measure the electrical impulse of movement is to use electromyography (EMG). EMG can record the electrical activity of muscles during movement, and this data can then be converted into patterns of electrical stimulation to mimic muscle movement.
An example of a movement that can be measured and reproduced using electrical impulses is the flexion-extension movement of the interphalangeal joint of the thumb.
